(1) Field of the Invention
The present invention generally relates to cylinder and reciprocating piston assemblies for use under high pressure in high ambient pressure environments and, more particularly, to such assemblies which are exposed over long periods of time to corrosive liquids such as sea-water.
(2) Description of the Prior Art
Many ocean-going vessels and submarines, in particular, commonly include movable structures which require hydraulically or pneumatically derived forces to be applied in order to achieve the desired motion, either due to the size or mass of the movable structure, the speed of motion or acceleration to be achieved, static or dynamic pressures resisting such motion or because of inaccessibility of the structure to personnel. In some cases, high pressure air or steam can-be directly applied to portions of the structure to develop necessary forces. In other cases, cylinder and piston assemblies driven by high pressure air or steam are required in order to contain high relative pressures or to maintain separation between the fluid used to generate the force and ambient fluids, such as sea water. In particular, in numerous structures common on submersible vessels, such as launchers for various payloads, depth of submersion of the vessel may impose extreme hydrostatic pressures against which pneumatic or hydraulic pressure must work. The piston assembly must also prevent penetration of sea water into the launcher or the vessel when actuating pressure is not applied.
It has been found that a particularly critical application for cylinder and reciprocating piston assemblies is for an impulse or power cylinder in a launcher employed on submarines. In this application, the piston and load to which it is connected must be rapidly driven by high pressure (generally derived from high-pressure compressed air) to a velocity of approximately one hundred inches per second or more over a relatively short distance of a few feet. Transfer of a sufficient amount of fluid to a cylinder at sufficient pressure to achieve such accelerations of a load and acting against potentially large ambient hydrostatic pressure requires a specially constructed firing valve to be employed.
Cylinders for such an application are currently machined from a copper-nickel (CuNi) alloy which is of sufficient strength to withstand the pressures involved without requiring an unacceptable mass of material and exhibits a degree of corrosion resistance. A piston preferably made of nickel-aluminum-bronze (Nixe2x80x94Alxe2x80x94Br) material, is arranged to ride within the inner bore of the cylinder. O-ring grooves, seals and other arrangements for preventing leakage of fluid past the piston within the bore of the cylinder are generally employed and the inner bore of the cylinder must be machined to a high degree of smoothness to prevent damage to the piston and seals. However, CuNi material is subject to crevice corrosion when in contact with sea water for extended periods of time. Such corrosion causes pitting of the inner bore of the cylinder. The pitted cylinder cannot be effectively sealed by structures provided on the piston and roughness due to such pitting may cause damage to the seals when the piston is moved.
Since the portion of the cylinder through which the piston must move is generally exposed to sea water and often at high hydrostatic pressures, as pitting increases, the piston becomes less effective in maintaining a separation of sea water from the portion of the inner bore of the cylinder to which pressure is applied. Leakage of sea water into this portion of the cylinder causes catastrophic failure of the firing valve. Failure of the firing valve will cause failure of a launch of payload apparatus which is potentially very expensive. Repair of the firing valve is also expensive and inconvenient. Repair at sea cannot generally be accomplished due to inaccessibility of the structure and the launch apparatus must generally remain non-functional until repairs can be accomplished.
Reworking the cylinder at the present state of the art has included the lining of the inner bore of the cylinder with a liner sleeve of CuNi material which is then machined to close tolerances to again prevent leakage past the piston. Other metal and alloy materials tend to accelerate the progress of corrosion and many cannot withstand the pressures and other severe operational conditions of the impulse cylinder and piston arrangement, such as the friction of the piston against the inner cylinder bore. However, as can readily be understood, the CuNi material of the liner sleeve is similarly subject to corrosion due to contact with sea water and the cycle of corrosion, leakage, catastrophic failure of the firing valve and replacement of the firing valve is repeated. Therefore, such corrosion presents a very substantial economic cost which has not been previously avoidable, particularly in the adverse conditions of the application and the extreme operating conditions of the cylinder and piston arrangement.
Providing corrosion protection for metal with a polymer coating is known. For example, U.S. Pat. No. 5,441,772 to McAndrew et al. teaches protection of carbon steel with nonconducting poly(aniline). U.S. Pat. No. 3,459,628 to Davis et al. teaches corrosion protection with a urethane foam composition and U.S. Pat. No. 3,012,710 to Steinacker teaches an elastomer liner for a centrifugal separator for corrosive liquids. U.S. Pat. No. 5,364,012 to Davis et al. and U.S. Pat. No. 3,738,527 to Townsend teach liners for liquid storage tanks which may be pressurized. However, such applications do not involve withstanding high impulse pressures with minimal distortion or resisting abrasion as would occur in a reciprocating piston and cylinder assembly.
Liners of metal are also known for piston and cylinder assemblies such as cast-iron liners in aluminum block internal combustion engines. However, in such an application, long-term exposure to a corrosive liquid is not generally involved or a degree of corrosion can be tolerated in view of ease of repair. Lubrication is also generally possible to increase resistance to abrasion and corrosion. However, such lubrication cannot be accomplished in the presence of long-term exposure to a corrosive liquid which will wash away any such material from the cylinder walls.
U.S. Pat. No. 5,348,425 to Heiliger discusses a French Patent Publication 1,202,536 which uses a thermoplastic material for lining a cylinder for a protective coating in a cylinder and piston assembly but notes that such coatings are permeable to oxygen and water and, if exposed thereto, form water and gas pockets at the interface of metal and the coating at which corrosion occurs. The gas or water pockets are driven along the interface between the metal and coating by the piston leading to peeling of the coating. In a mine prop, to which the Heiliger patent is directed, the thermoplastic coating would fail in such a way. Additionally, since mine props require a pressure differential to be applied across the piston for extended periods of time, a step deformation occurs due to the radial elasticity of the thermoplastic coating material. This step deformation damages ring seals which are used on the piston.
To avoid such deformation and other problems, Heiliger proposes the use of a three-dimensionally cross-linking thermosetting coating of 150-250 xcexcm thickness on the cylinder interior and the exterior of the piston. However, the advantages gained by Heiliger in the application to a mine prop are achieved by reduction of the elasticity of the coating. Such an approach may be acceptable in such an application in which pressure is applied for long periods of time and changes in pressure are gradual but is not suitable for extreme impulse pressures. Also, in such an application, the resistance of such a coating to abrasion is of relatively little importance since piston velocity is very low. Further, in Heiliger and the French Patent Publication as described therein, the corrosion resistant material is applied as a coating to smooth the inner bore of the cylinder and reduce machining thereof as well as to achieve good adherence to high strength steel which is particularly subject to damage by corrosion. A coating, by its method of application (even if as a preformed sleeve) and, in the case of Heiliger, in-situ curing cannot achieve the high degree of structural integrity required when high impulse pressures are repeatedly applied, as in an internal combustion engine or an impulse cylinder for a payload launcher in a submersible vessel described above.
Accordingly, there has been no structure heretofore known which would simultaneously provide resistance to corrosion due to long-term exposure to corrosive and high-pressure liquids, capable of withstanding high impulse pressures (for example, 560 to 1350 psi above ambient pressure in the preferred impulse cylinder application) and the abrasion incident to high acceleration and speed of a piston and highly effective and reliable for maintaining a separation between the corrosive fluid and other structures.
It is therefore an object of the present invention to provide a sea water resistant, corrosion resistant, non-metallic liner for a sealing surface of a reciprocating piston and cylinder arrangement.
It is a another object of the invention to provide an economical and simplified method of fabricating or reworking a reciprocating piston and cylinder arrangement to achieve a corrosion resistant, non-metallic sealing surface.
It is a further object of the invention to provide a reciprocating piston and cylinder arrangement for a launching mechanism which avoids damage and/or failure of valves therein and improves usefulness and reliability of the launching mechanism.
It is yet another object of the invention to provide a cylinder and reciprocating piston assembly which is highly reliable and effective for maintaining a separation of corrosive fluids from structures exposed to the interior of the cylinder.
In order to accomplish these and other objects of the invention, a cylinder is provided for or together with a cylinder and reciprocating piston assembly including a metallic outer cylinder having an inner bore and an elastomer sleeve liner within the inner bore of the outer cylinder and compressionally preloaded in a radial direction about the circumference of the liner by the outer cylinder.
In accordance with another aspect of the invention, a method for making a corrosion-resistant cylinder is provided including the steps of placing a molded urethane elastomer liner within an inner bore of a rigid outer metallic cylinder, an outer diameter of the molded urethane elastomer liner being slightly larger than a diameter of the inner bore of said rigid outer metallic cylinder at an ambient temperature, the outer diameter of the molded urethane elastomer liner decreasing with decreasing temperature and the diameter of the inner bore of the rigid outer metallic cylinder increasing with increasing temperature, and preloading the molded urethane elastomer liner with the outer cylinder at an ambient temperature.